1) Field of the Invention
This invention relates generally to fabrication of semiconductor devices and more particularly to the fabrication of fully dielectric isolated silicon.
2) Description of the Prior Art
It is often necessary for devices constructed in the same substrate to be electrically isolated from the substrate and from each other to prevent undesired interactions between them. Dielectric structures are used to provide such isolation. Various methods of forming isolation structures have been disclosed. One method of forming isolation structures is SIMOX (Separation by Implanted Oxygen). SIMOX forms a buried dielectric oxide layer by implanting oxygen ions at a particular energy level. However, a large thermal budget is required for high temperature annealing to repair the damage to the silicon layer overlying the buried oxide layer caused by implanting oxygen ions. Also, with SIMOX, a separate isolation structure must be formed between devices.
Another method for forming isolation structures comprises using an HF anodizing step to form porous silicon, then oxidizing the porous silicon. Because the HF anodizing step is highly selective to doped silicon, a porous silicon pattern can be formed without harming areas of undoped silicon where devices are to be formed. A common method of forming buried porous silicon to isolate device areas from the substrate is epitaxial growth of monocrystalline silicon over a layer of porous silicon that will subsequently be anodized. However, to prevent damage to the monocrystalline silicon, the epitaxial silicon must be formed at low temperatures resulting in a lower quality monocrystalline structure.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering the following patents.
U.S. Pat. No. 3,919,060 (Pogge et al.) shows a method for dielectrically isolating silicon regions grown on a substrate using anodic etching and oxidation.
U.S. Pat. No. 4,016,017 (Aboaf et al.) shows a method for forming dielectric regions by oxidizing and densifying porous silicon on a substrate surface.
U.S. Pat. No. 4,393,577 (Imai), U.S. Pat. No. 4,532,700 (Kinney et al.), and U.S. Pat. No. 4,627,883 (Holmstrom et al.) show methods of forming devices using anodic etches.
U.S. Pat. No. 4,506,283 (Soclof) shows a method of forming resistors using an anodic treatment.
U.S. Pat. No. 5,023,200 (Blewer et al.) shows a method for forming isolated silicon regions over a conductive layer by oxidizing and densifying porous silicon, but does not provide the planarized fully dielectrically isolated silicon of the present invention.